Fiber sintering



2 Sheets -Sheet 1 A ril 13, 1965 s. w. M GEE ETAL FIBER SINTERING Filed Nov. l4, 1962 1 1 1 1 1 1 1 1 1 1 1 1 1 1 11 11111/ 1 1 1,111 011,111 1 O o/1 1 12 wa /1 A Nm L 3 R v. :w as 3 Xxx 3w 3 xxx 0N wN I I/ l l 1 1 1 ,1 1 1 1 1 1 1 1 1 01 1 1M 1 11 H 11H 1 1 1 1 11 1 m- 1wn- 7 11 11 1111 11 1 11 11 1 11H 1 1 1 1 1 H H 1 1 fl 1 111 n 1 1 1 1 1 1 1 1 1 un 1 1 1111 11 1 1 1 11 1 1 111 1 11 /1111 1 1 1 1 1 1 1111 1 1 11111 11 1 1 1 1, 111 1 11 1 1 11 11 1 11 1 1 1 1 11 1 1 1 1 1 11 1 1111 1 111 1 1 1 1 1111 1 1 11 1 1 1 1 1 1 11 1 1 1 1 1 11111 1 1 111 1 1 1 1111/11 11 1 1 11 1 1 1 1 11 11 1 1 1 1 1 1 1 11111 1 1 1 1 11 1 1 1 1 11111 1 1 1 1 111 1 1 1 1 1 11 111 1 1 111 11111/111 a 1 1 111 11 1 1111 1 11 1 11 11 1 1 1 111 1 11 11 1 1 11 1 11 1 1 1 1 1 1 1 1 1 1 1 11 1 1 1 111 1 1 111 1 11 11 1 1 111 1111 1 11 11 1 11 1 1 11 11 1 11 1 1 1 1 1 1 1 1 1 11 11 11 11 1 1 11 111 1 11111 1 1 1 7 1 11 ,114 1 a April 13, 1965 s. w. MOGEE ETAL FIBER SINTERING 2 Sheets-Sheet 2 Filed NOV. 14, 1962 FIG.3.

United States Patent Office 3,178,280 Patented Apr. 13, 1965 3,178,280 FIBER SINTERING Sherwood William McGee, Lisle, Ill., and James Irwin Fisher, Orange, Conn, assignors, by mesne assignments,

to Huyclr Corporation, New York, N.Y., a corporation of New York Filed Nov. 14, 1962, Ser. No. 237,496 8 Claims. (Cl. 75-200) This invention relates to metal fibers and, more particularly, to porous, sintered bodies or masses of such fibers and to processes for forming such bodies.

Fiber metallurgy has, as one of its objects, the production of very highly porous sintered metal bodies. Such bodies are produced by stacking metal fibers through felting to produce a body with much lower apparent density than can be produced from other types of metallic particle. In fiber metallurgy, it is common practice to provide these low density felted bodies with an appreciable integrity and strength by giving the bodies a sintering treatmerit at an elevated temperature. In essence, sintering results from inter-fiber diffusion, forming metallic bonds at those points where the fibers make metal-to-metal contact.

In practice, difficulties often arise in achieving satisfactory metal-to-metal contact between metal fibers and in Obtaining sufiicient diffusion to produce satisfactory bonding even though metal-to-metal contact may exist.

Certain metals, such as aluminum and beryllium, have oxides with high free energes of formation, which are difficult to reduce at temperatures below the melting point of the metals themselves. Because tightly-adhering oxides are formed on the surface of these metals on exposure to air, when sintering is attempted with fibers of such metals, the oxide forms an irreducible barrier which inhibits the abutting particles from joining by interrnatallic difiusion.

Certain other metals, such as, cadmium, tin, lead, and the like, although not inhibited from sintering by irreducible oxides, possess another characteristic which renders their sinter bonding extremely difficult if not impossible. This characteristic is that their melting point is sufliciently low that when attempts are made to sinter them at temperatures below their melting points, their low self-diffusion rates make the necessarily excessive holding times at temperature impractical. Such metals are therefore usually considered to be not sinterable.

It is sometimes possible to effect sintering by adding alloying elements to the metal which improve its sinterability. Of course, such a process alters the nature of the pure metal and will often not be satisfactory for this reason. Another method of sintering such metals is by the addition of a bonding metal which may be added as a coating to the fibers. In such cases, the fiber metal body is heated to a temperature above the melting point of the bonding metal to elfect a braze at the fiber junctions. The resulting sintered fiber metal body differs substantially in character from that of the basic metal.

It is an object of the instant invention to provide an improved process for sintering metal fibers, particularly in low density or highly porous bodies.

It is a further object to provide such a process in which sintering can be accomplished despite surface coating or films on the fibers or the proximity of the sintering temperature to the melting temperature of the fibers.

Still a further object of the invention is to provide a process wherein the bond area between the fibers is increased pr-ior to the metallic diffusion.

Another object of the invention is to provide such a process in which the fibers are bonded by diffusion of the metal of the fiber without resorting to pretreatment of the fibers or the addition of bonding metals or fluxes.

These and other objects will be more readily apparent from the following description and attached drawing in which FIG. 1 is a side elevation view, partly in section, of an apparatus for carrying out the process of the instant invention;

FIG. 2 is a side elevation View, partly in section, of a further apparatus for carrying out the instant invention; and

FIG. 3 is a side elevation view, partly in section, of an apparatus for carrying out the instant invention on a continuous stream of fiber bodies.

Under the present invention the difficulties encountered prior hereto in sintering metal fibers which oxidize and metal fibers having diffusion and melting temperatures which are proximate, are avoided. It has been discovered that such metal fibers can be sintered by vibrating the fibers under controlled conditions. In carrying out the process of the instant invention, the fibers are felted and, while held in contact with each other, are vibrated. During vibration, adjoining fibers are caused to rub against each other. Such rubbing abrades and frets the abutting surfaces of the rubbing fibers, disrupting any oxide or film which might be present between the fibers in the rubbing or abrading area. Since the fibers are rubbing against each other, friction raises or elevates the temperature at those sites where the fibers are in contact. Thus, the elevation in temperature occurs at the sites where diffusion is to occur. Diffusion and sintering are, therefore, accelerated.

Vibration, as contemplated by the instant invention, also improves diffusion or sintering in yet another manner. The strength of the sintered or diffused bond is, to a large extent, governed by the size of the area over which diffusion or sintering takes place. Considering the joint formed by two round fibers intersecting at right angles, the contact between the two round fibers is, for all practical purposes, a point. Thus, when such fibers are sintered in the normal manner, initial bonding occurs at this point and a substantial amount of time at the sintering temperature is required to permit the diffusion bond to develop and produce a bond comparable in its cross sectional dimensions to the dimensions of the fibers. On the other hand, in the instant invention the two fibers are vibrated and rubbed together before diffusion or sintering occurs. Prior to diffusion, each fiber rubs on the other fiber or particle a flat surface and diffusion or sintering subsequently occurs in such flat surface. Consequently, when diffusion does occur the area of contact between the fibers is large, producing a bond of relatively large cross section immediately. Thus, the time at sintering temperature is reduced.

Vibration may be applied to the felted fiber body by introducing a mechanical compression wave traveling through the body. Such a compression wave can be induced in many ways, one of which is to alternately compress and rarify the body to a slight extent by a simple mechanical drive. Rarefaction is accomplished by virtue of the fact that the fibers are not deformed beyond their elastic limit during compression even at the elevated temperature and are thus able to spring back into their original position when the compresison force is removed each half cycle. Rather than mechanically, the compression wave can be induced magnetically, or sonically.

.3: A low frequency vibration which will not be lost in the first layers of the felt through poor impedance match between the vibration source and fibers of the metal felt body is prefered. For example, ultrasonic vibration sources do not generally produce the desired inter-fiber action and should be avoided. On the other hand, sonic frequencies of from 40 to 20,000 cycles/sec. and subsonic frequencies below 40 cycles/sec. are satisfactory to produce the required inter-fiber action.

Preferably, the fibers while being sintered are in a reducing atmosphere, or an atmosphere of inert gas, or a vacuum, all being selected to minimize or prevent the formation of deleterious oxides. Inert gases such as argon, nitrogen or helium may be used, and reducing atmospheres found useful are dry hydrogen and cracked ammonia.

It is additionally important that to effect a satisfactory bond, the amplitude of the vibration be not so great that the fibers are grossly moved from their starting location, nor so great that a bond cannot occur at the propitious moment. Optimum amplitude will vary with the material being sintered, sintering temperature, and the like. VJith aluminum, for example, it has been found preferable that the amplitude not exceed two fiber diameters at normal sintering temperatures. On the other hand, in order to rapidly establish an enlarged bonding area, a temporary vibrational amplitude greater than optimum conditions may be beneficial in producing a large abraded area. However, once the desired diffusion conditions are established, the vibration amplitude should be decreased if proper sintering is to be achieved.

Referring now to the drawings, in the apparatus of FIG. 1 a loose mass 2 of metal fibers are held cornpacted at one end of the tube 4 by plate 6, urged against the fiber mass 2 by one end of compression spring 18. At its other end, spring 18 is compressed by anvil 12, held in place in tube 4 by rod 14 and stop 16 mounted in recess 18 of tube 4. The end of tube 4 is closed by stopper 26, the interior of the tube being connected by conduit 22 passing through stopper 2d and connected to a gas supply tank, through valve 23, shown diagrammatically at 24. Conduit 22 is also connected through valve 25 to a pump shown diagrammatically at 27.

The inner or closed end of tube 4 is connected by clamp 26 to vibrator 28, which may be of any conventional type adapted to provide vibration in clamp 26 and tube 4 at controlled amplitude. The inner end of tube 4 containing fiber mass 2 is positioned in a suitable heating furnace 30 having on opposite sides of tube 4, heating coils 32.

In using the apparatus of FIG. 1, loose fibers to be sintered are first placed in tube 4 and plate 6 and spring 19 are inserted in the tube. Anvil 12 is next inserted, the inner end of the anvil contacting the end of spring 18, compressing the spring and lightly compacting the loose fibers in the inner end of tube 4. Stop 16 is positioned against the outer end of anvil 12 and inserted in recess 18 to hold anvil 12 in place, spring compressed and the fibers in the tube lightly compressed. The assembled tube, with the compact loose fibers, is then positioned in furnace 30, the inner end of tube 4 being inserted into clamp 26 of vibrator drive 28. Stopper 20 and conduit 22 are next inserted into the open end of tube 4. Clamp 26, stopper 20 and conduit 22 suspend tube 4; in place in furnace 30.

With tube 4 suspended in furnace 38, valve 23 is closed and valve 25 is opened. Pump 27 evacuates tube 4 and valve 25 is closed. Valve 23 is then opened delivering gas from tank 24 to tube 4-. As tube 4 is being evacuated and filled with gas, or immediately thereafter, furnace 3t) and coils 32, raise the temperature of metal fibers in the end of tube 4 to a temperature just below the temperature at which sintering or diffusion of the fibers is to take place. As the temperature of the fibers is raised, vibrator 28 is actuated to vibrate clamp 26 and the fibers in tube 4. Heating of the fibers and vibration is continued until the fiber mass has been subjected to sufficient compressive waves that the necessary inter-fiber abrasion has occurred at the points of intersection. When sufficient abrasion has occurred, a bond nucleus will form and abrasion will cease. Subsequently, diffusion between the metal fibers continues and strengthens the bond over the contact area.

Referring now to FIGS. 2 and 3, there are shown other apparatus for carrying out the process of the instant invention. In FIG. 2 furnace 50, having heating coils 52, is provided with a recess 54. Housing 56, closed at its lower end, is positioned in recess 54. Cover 58 is attaohed to the open end of housing 56 by bolts 60. A sealing ring 62 is positioned between housing 56 and cover 58, forming an airtight seal between the housing and cover.

A port 64 in cover 58 is connected, by conduit 66 and valve 68, to a compressed gas supply tank, shown diagrammatically at 78. Conduit 66 is connected by valve '72 to pump 74. Bushing 76 is fitted into the lower end of housing 56 with heat shield 78, having an opening 80, positioned on top of bushing 76. Vibrator 82, equipped with means for controlling vibration amplitude and of conventional design, is supported above housing 76 by resonator spring 84 and bellows 86, bellows 86 forming a gastight connection between vibrator 82 and cover 58. Vibrator 82 is connected to anvil 88, anvil 88 extending downwardly into housing 56 and having, at its lower end, an anvil plate 98.

In use, a mass of loose fibers are positioned in the bottom of housing 56 inside of bushing 76. Vibrator 82 and anvil 88, through anvil plate 90, induce low amplitude compression waves through the mass of loose fibers, producing the desired inter-fiber abrasion.

With fibers 92 in position and being abraded through the action of the vibrator 82, anvil 88 and anvil plate 90, housing 56 is evacuated through port 64 and conduit 66 by opening valve 72 and actuating pump 74, or by purging the housing with a suitable gas. Valve 68 is, of course, closed. After the air is evacuated or purged, valve '72 is closed, valve 68 is opened and gas from cylinder 78 is delivered to housing 56 through conduit 66 and port 64.

A housing 56 is evacuated and filled with gas or immediately thereafter, furnace 5t) and coils 52 raise the temperature of the fiber mass 92 to a temperature at which sintering and diffusion of the fibers is to take place. Vibrator 82 is actuated and, through anvil 88 and anvil plate 98, vibrates the fibers in fiber mass 82. Vibration of the fibers is continued until the fibers are sintered. Either throughout the operation or at least at the time the fibers are being sintered, the amplitude of vibration of fibers 92 by vibrator 82 through anvil 88 and anvil plate 98 should be regulated to optimum amplitude conditions.

In FIG. 3 another apparatus is shown for carrying out the process of the instant invention. In this apparatus the fiber masses are sintered on a semi-continuous basis, that is, unsintered bodies of loose fibers are formed, cornpacted on a pallet and, on the pallet, are passed into a sintering zone where the fibers are sintered. The sintered mats are then passed out of the zone and another pallet is inserted.

Referring to FIG. 3, there is shown a furnace 1%) having base 104 and heating coils 102. Vibration table 186 is connected through base 104 by legs 108 to a vibrator 110. Vibrator 110 is of a type in which the amplitude of the vibration on the vibrating table can be controlled in known manner. Preformed mats or blocks 112 of loose fibers are positioned on pallets 114, and plate 116 of suitable weight is positioned on the top of each mat or block 112 to lightly compact and hold the block.

In carrying out the process with this apparatus, preformed blocks of unsintered fibers are positioned on pallets 114. Plate 116 is then positioned on the block. The pallet is then fed into furnace 100 and into position on vibrator table 106. When in position on table 106, the block 112 is heated by the furnace and, at the same time, vibrated by vibration table 106 driven by vibrator 110 through legs 108. As heretofore noted, at the time sintering is to occur in the block, the vibratory drive is regulated so that the vibration imparted to the fibers in the block does not substantially exceed a fiber diameter. In this case, the compression wave is induced in the felted fiber body by virtue of the force imposed by the upward acceleration of the mass on top of the fiber body. During the downward stroke, this force is of course reduced and the rarefaction portion of the cycle is then obtained. After the fibers have diffused and are sintered, pallet 114 with its sintered fiber block is removed from the vibration table and a second pallet is positioned on the table.

In FIG. 3, the apparatus is shown for use where diffusion and sintering is to be performed under atmospheric conditions. With some types of metal fibers sintering can be effectively accomplished under the instant process under such conditions. Where, however, the metal fibers undergoing treatment require an evacuated or inert atmosphere, it is to be understood that the apparatus may be enclosed and provided with suitable means, similar to those shown in FIGS. 1 and 3, for evacuating the sintering zone and, if required, supplying an atmosphere of inert gas to such Zone.

In the following examples metal fibers were sintered employing the process of the instant invention. The apparatus of FIG. 1 was employed in these examples.

Example 1 0.010 in. aluminum wire was cut into lengths and compacted in the inner end of the tube. The tube was evacuated to a vacuum of 6 10* ml. and heated to a temperature of 1200-1205 F. The lightly compact mass was vibrated at 7200 c.p.m. and at a vibration amplitude of 0.015".

The fiber mass was removed and found to be well sintered.

Example 2 The procedure of Example 1 was repeated but the temperature was regulated to ll901l95 F. When removed, the fiber body was well sintered.

Example 3 The procedure of Example 1 was repeated using chemically pure cadmium metal fibers having a nominal size of 0.060". The fibers were felted into a body /s in diameter and 1%" long and heated to a temperature of SSS-500 F. The felted, heated body was vibrated at 7200 c.p.m. for A hour. At the end of 1 hour, vibration was terminated and the fibers were held at 585-600 F. for 45 minutes.

The fiber mass was removed and found to be strongly sintered and there was no evidence of melting.

Example 4 The procedure of Example 3 was repeated, the vibration was omitted, and the fibers were held at a temperature of 590600 F. for 2%. hours. At the end of 2% hours the fiber body was removed and little, if any, sintering could be found.

While the foregoing description has been directed to metals, such as, aluminum, cadmium, lead, tin, and zinc, it is to be understood that other metals which, heretofore, have been sintered by other methods, can be sintered using the vibration technique of the instant invention. As has been noted above, during vibration the fibers abrade and form flat surfaces which abut at the area where diffusion is to occur. These fiat surfaces substantially increase the diffused area and increase the bond between fibers. Thus, even though the metal fibers might be sinterable by other methods, improvement in strength can be expected when such metals are sintered by the present method.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

What is claimed is:

1. A process for sintering metal fibers by diffusion the steps comprising, heating a felted mass of loose fibers to a temperature at which diffusion is to occur and, While holding said heated felted mass lightly compressed, applying a vibratory force to said mass to cause adjoining fibers to rub against each other and to abrade each other at their abutting surfaces, thereafter, diffusing the metal and sintering said fibers at their abraded, abutting surfaces.

2. A process for sintering metal fibers by diffusion the steps comprising, lightly compressing a mass of loose fibers, heating said compressed mass to a temperature at which diffusion is to occur and, while said fibers are heated to said temperature and lightly compressed, applying a vibratory force to said mass to cause adjoining fibers to rub against each other and to abrade each other at their abutting surfaces, thereafter, diffusing the metal and sintering said fibers at their abraded, abutting surfaces.

3. A process for sintering metal fibers by diffusion the steps comprising, lightly compressing a mass of loose fibers, heating said mass to a temperature at which diffusion is to occur and, while said fibers are heated to said temperature and lightly compressed, applying a vibratory force to said mass and, by inter-fiber abrasion, forming diifusable areas on abutting fibers and diffusing the metal of said fibers at said areas.

4. A process for sintering fibers of a metal selected from the group consisting of aluminum, cadmium, lead, tin, and zinc by metal diffusion, the steps comprising, heating a mass of loose fibers to a temperature at which diffusion is to occur, while said fibers are heated to said temperature, applying a vibratory force to said fibers and forming, by inter-fiber abrasion, dilfusable areas between abutting fibers and, after said areas are formed, maintaining said fibers at said temperature until diffusion bonds are formed between said fibers at said areas.

5. A process for sintering fibers of a metal selected from the group consisting of aluminum, cadmium, lead, tin, and zinc by metal diffusion, the steps comprising, lightly compressing a mass of loose fibers, heating said mass to a temperature at which diifusion is to occur, while said fibers are heated to said temperature, applying a vibratory force to said fibers and forming at the abutting interfaces of said fibers, by inter-fiber abrasion, areas for said fibers to diffuse and, after said areas are formed, maintaining said fibers at said temperature until diffusion bonds are formed between said fibers at said areas.

6. A process for sintering fibers of a metal selected from the group consisting of aluminum, cadmium, lead, tin, and zinc by metal diffusion, the steps comprising, felting a mass of loose fibers, heating said felted mass to a temperature at which diffusion is to occur and, while said fibers are heated to said temperature, applying a vibratory force to said fibers and forming at the abutting interfaces of said fibers, by inter-fiber abrasion, diffusion areas and, after said diffusion areas are formed, maintaining said fibers at said temperature until the metal in said fibers diffuse and said fibers sinter.

7. A process for sintering aluminum fibers by diffusion the steps comprising, felting a mass of loose fibers, heating said felted mass to a temperature at which diffusion is to occur and, while said fibers are heated to said temperature, vibrating said fibers to form at the fiber interfaces, by inter-fiber abrasion, diffusion areas and after said diffusion areas are formed, maintaining said fibers at 7 8 said temperature until the metals in said fibers diffuse and References Cited by the Examiner said fibers sinter. F

8. A process for sintering cadmium fibers by diffusion UNIITDD STATES PATENTS the steps comprising, felting a mass of loose fibers, hea 2,234,127 3/41 Mautsching said felted mass to a temperature at which diffusion 2,522,082 9/50 Arnold 75212 is to occur and, While said fibers are heated to said tern- 2,903,787 9/59 Brennanperature, vibrating said fibers to form at the fiber inter- FOREIGN PATENTS faces, by inter-fiber abrasion, diffusion areas and, after 821,690 10/59 Great Britain said diffusion areas are formed, maintaining said fibers at said temperature until the metal in said fibers difi'use 10 CARL 'D-QUARFORTH, Primary Examine"- and said fibers sinter. REUBEN EPSTEIN, Examiner. 

1. A PROCESS FOR SINTERING METAL FIBERS BY DIFFUSION THE STEPS COMPRISING, HEATING A FELTED MASS OF LOOSE FIBERS TO A TEMPERATURE AT WHICH DIFFUSION IS TO OCCUR AND, WHILE HOLDING SAID HEATED FELTED MASS LIGHTLY COMPRESSED, APPLYING A VIBRATORY FORCE TO SAID MASS TO CAUSE ADJOINING FIBERS TO RUB AGAINST EACH OTHER AND TO ABRADE EACH OTHER AT THEIR ABUTTING SURFACES, THEREAFTER, DIFFUSING THE METAL AND SINTERING SAID FIBERS AT THEIR ABRADED, ABUTTING SURFACES. 